PREPARATION OF BUFFERS To make an effective buffer
PREPARATION OF BUFFERS To make an effective buffer with a specific p. H, choose an acid with a p. Ka as close as possible to the required p. H Acetic acid has a Ka = 1. 8 x 10 -5 ∴ ∴ p. Ka = 4. 74 Acetic acid and the acetate ion can be used to make buffers from p. H = 3. 74 to p. H = 5. 74 EX 7 -1 (of 9)
Prepare a p. H = 9. 50 buffer by mixing 50. 0 m. L of a weak acid solution with the correct volume of a strong base solution Chemicals used: (1) weak acid (2) strong base p. H = p. Ka + log (nweak base + nstrong base) ______________ (nweak acid – nstrong base) EX 7 -2 (of 9)
Prepare a p. H = 9. 50 buffer by mixing 50. 0 m. L of a weak acid solution with the correct volume of a strong base solution Chemicals 0. 1000 M HCl (found on the side counter) 1. 000 M Na. OH 0. 40 M NH 4 Cl 0. 40 M Na. H 2 PO 4 Solid Na. C 2 H 3 O 2. 3 H 2 O 0. 1 M Na. H 2 PO 4/0. 1 M Na 2 HPO 4 buffer Acid H 3 PO 4 H 2 PO 4 HPO 42 HC 2 H 3 O 2 NH 4+ EX 7 -3 (of 9) Ka 7. 6 x 10 -3 6. 2 x 10 -8 2. 1 x 10 -13 1. 8 x 10 -5 5. 6 x 10 -10 p. Ka 2. 12 7. 21 12. 68 4. 74 9. 25 Correct weak acid Closest to 9. 50
Prepare a p. H = 9. 50 buffer by mixing 50. 0 m. L of a weak acid solution with the correct volume of a strong base solution CALCULATION 1 – Moles of weak acid in the buffer weak acid : 0. 40 M NH 4+ from 0. 40 M NH 4 Cl 0. 40 mol NH 4+ x 0. 0500 L solution _________ L solution EX 7 -4 (of 9) = 0. 020 mol NH 4+
Prepare a p. H = 9. 50 buffer by mixing 50. 0 m. L of a weak acid solution with the correct volume of a strong base solution CALCULATION 2 – Moles of strong base in the buffer p. H = p. Ka + log (nweak base + nstrong base) ______________ (nweak acid – nstrong base) p. H : 9. 50 p. Ka : 9. 25 weak base : 0 mol NH 3 weak acid : 0. 020 mol NH 4+ strong base : x mol OH- EX 7 -5 (of 9)
Prepare a p. H = 9. 50 buffer by mixing 50. 0 m. L of a weak acid solution with the correct volume of a strong base solution 9. 50 = 9. 25 + log (0 mol + nstrong base) _______________ (0. 020 mol – nstrong base) 0. 25 = log nstrong base _______________ (0. 020 mol – nstrong base) 1. 778 = nstrong base _______________ (0. 020 mol – nstrong base) EX 7 -6 (of 9)
Prepare a p. H = 9. 50 buffer by mixing 50. 0 m. L of a weak acid solution with the correct volume of a strong base solution 1. 778 = nstrong base _______________ (0. 020 mol – nstrong base) 1. 778 (0. 020 mol – nstrong base) = nstrong base 0. 03557 – 1. 778 nstrong base = nstrong base 0. 03557 = 2. 778 nstrong base 0. 01280 = nstrong base EX 7 -7 (of 9)
Prepare a p. H = 9. 50 buffer by mixing 50. 0 m. L of a weak acid solution with the correct volume of a strong base solution Chemicals 0. 1000 M HCl (found on the side counter) 1. 000 M Na. OH 0. 40 M NH 4 Cl 0. 40 M Na. H 2 PO 4 Solid Na. C 2 H 3 O 2. 3 H 2 O 0. 1 M Na. H 2 PO 4/0. 1 M Na 2 HPO 4 buffer Acid H 3 PO 4 H 2 PO 4 HPO 42 HC 2 H 3 O 2 NH 4+ EX 7 -8 (of 9) Ka 7. 6 x 10 -3 6. 2 x 10 -8 2. 1 x 10 -13 1. 8 x 10 -5 5. 6 x 10 -10 p. Ka 2. 12 7. 21 12. 68 4. 74 9. 23 Only strong base
Prepare a p. H = 9. 50 buffer by mixing 50. 0 m. L of a weak acid solution with the correct volume of a strong base solution 0. 01280 mol Na. OH x L solution ___________ 1. 000 mol Na. OH EX 7 -9 (of 9) = 0. 013 L = 13 m. L
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